Number of moles of cobalt can be calculated as given mass of the substance divided by the molar mass of the substance.
Here, given mass of the cobalt = 27.4 g
Molar mass of the cobalt = 58.9 g
Number of moles of a substance= \frac{Given mass of the substance}{Molar mass of the substance}
Number of moles of cobalt= \frac{Given mass of the cobalt}{Molar mass of the cobalt}
=\frac{27.4}{58.9}
=0.465 mol
So, number of moles of cobalt will be 0.465 mol